Metal-ligand cooperation (MLC), in which both the metal and the ligand undergo bond-making and breaking with incoming substrates, plays an important role in chemical and biological catalysis. A prevailing mode of MLC, is based on metal cooperation with ligands bearing N—H groups, mostly of the form H-M-NHR (Scheme 1a). Among several systems that operate in this fashion, pincer-type ligands have recently shown exceptional catalytic activity in hydrogenation and dehydrogenation reactions involving carbonyl groups. There are examples for catalytic hydrogenation of aldehydes, esters, imines, ketones, nitriles, and even amides and cyclic carbonates based on such MLC. Dehydrogenative coupling of alcohols to esters, alcohols with amines to form amides, as well as other catalytic reactions have also been reported.

The inventors of the present invention previously developed a new mode of MLC, involving aromatization/dearomatization of pyridine and bipyridine based ligands (Scheme 1b, FIG. 1). For example, deprotonation of the benzylic proton α to the pyridine group in a pyridine-based pincer complex can lead to its de-aromatization. The de-aromatized moiety can regain its aromaticity by cooperation between the metal and the ligand, resulting in the activation of chemical bonds, such as H—X (X═NR2, H, OR or C), without formal change in the metal oxidation state (Scheme 1b). Several new, environmentally benign reactions catalyzed by PNP— or PNN—Ru and Fe pincer complexes, based on aromatization/de-aromatization strategy have been developed. They include dehydrogenative coupling of alcohols to esters, hydrogenation of esters to alcohols, coupling of alcohols with primary amines to form amides with liberation of H2, direct synthesis of imines from alcohols and amines with liberation of H2, catalytic coupling of nitriles with amines to selectively form imines and several other catalytic transformations.
Zeng et al. [Zeng, H.; Guan, Z. J. Am. Chem. Soc. 2011, 133, 1159] describes a process for preparation of polyamides via catalytic dehydrogenation of diols and diamines using PNN pincer ruthenium complexes.
U.S. Pat. No. 8,178,723, to some of the inventors of the present invention, describes methods for preparing amides, by reacting a primary amine and a primary alcohol in the presence of Ruthenium complexes, to generate the amide compound and molecular hydrogen.
U.S. Pat. No. 8,586,742, to some of the inventors of the present invention, describes methods for preparing primary amines from alcohols and ammonia in the presence of Ruthenium complexes, to generate the amine and water.
PCT patent publication no. WO 2012/052996 (U.S. Pat. No. 9,045,381) to some of the inventors of the present application, describes methods of using Ruthenium complexes for (1) hydrogenation of amides to alcohols and amines; (2) preparing amides from alcohols and amines; (3) hydrogenation of esters to alcohols; (4) hydrogenation of organic carbonates to alcohols and hydrogenation of carbamates or urea derivatives to alcohols and amines; (5) dehydrogenative coupling of alcohols to esters; (6) dehydrogenation of secondary alcohols to ketones; (7) amidation of esters (i.e., synthesis of amides from esters and amines); (8) acylation of alcohols using esters; (9) coupling of alcohols with water to form carboxylic acids; and (10) dehydrogenation of beta-amino alcohols to form pyrazines.
Given the widespread importance of amines, alcohols, amides and esters and their derivatives in biochemical and chemical systems, efficient syntheses that avoid the shortcomings of prior art processes are highly desirable.